Abundance Sensitivity of the Phoenix and Phoenix X62 TIMS

Zenon Palacz, IsotopX Ltd, Middlewich, Cheshire, UK.

Introduction  

Abundance sensitivity is a measure of the contribution of the peak ‘tail’ of a major isotope on an adjacent minor isotope. The tailing is due to the collision of ions with residual gas molecules in the mass spectrometer causing a loss of ion energy. This is manifest on a mass spectrum as a loss in mass giving rise to a ‘tail’ on the low mass side of a peak. Hence there is a direct correlation between abundance sensitivity and the quality of the analyser vacuum.  

Additionally, since any peak tail contribution from a major isotope will be proportional to its beam size, the associated errors will produce an apparent non-linearity in the measured ratio. Hence it is crucial that an MS system has low inherent abundance sensitivity and that an effective correction can be made for any small contribution that remains.  

By convention, abundance sensitivity is referred as the contribution of the ²³⁸U peak on mass 237 where there are no U isotopes, e.g 237/²³⁸U = 5e^-6 or 5ppm. In U analysis the abundance sensitivity is of particular importance since the 236U isotope of natural samples is less than 1ppm of the ²³⁸U peak so that even a small amount of ²³⁸U peak tailing down to m/z 236 is significant.  

Experimental  

We have used the standard U030a to illustrate the effect of abundance sensitivity on the Phoenix Thermal Ionization Mass Spectrometer. The certified isotopic composition of U030a is shown in Table 1. 

Figure 1 shows a mass scan on the axial ion counting Daly detector from m/z 233.5 to mass 237.5. The peak on the left is the ²³⁴U isotope which has an isotopic abundance of 287ppm of the ²³⁸U which is off scale to the right of this figure. The ²³⁵U peak is off scale on this scan, whilst the ²³⁶U peak with an abundance of 5.86ppm relative to the ²³⁸U is very small. The flat top of the ²³⁶U is clearly apparent, and the peak itself is readily resolvable from the baseline.  

To the right of the ²³⁶U peak there is a clear uplift in baseline due to the peak tail from the ²³⁸U. This is shown in more detail in Figure 2 (over). 

Figure 2. shows that the ²³⁸U peak tail extends very slightly beneath the ²³⁶U peak. At mass 237 the tail is less than 1/3 that of the ²³⁶U peak, and the abundance sensitivity is 1.8ppm. The analyser vacuum was <2e-9 mbar. 

Summary

The abundance sensitivity of the Phoenix TIMS is <2ppm at mass 237 with respect to ²³⁸U.

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